PHYSICAL REVIEW C 76, 027603 (2007) Experimental investigation of fusion of 7 Li+ 28 Si above the Coulomb barrier Mandira Sinha, 1,2 H. Majumdar, 1,* R. Bhattacharya, 2 P. Basu, 1 Subinit Roy, 1 M. Biswas, 1 R. Palit, 3 I. Mazumdar, 3 P. K. Joshi, 3 H. C. Jain, 3 and S. Kailas 4 1 Saha Institute of Nuclear Physics, 1/AF, Bidhan Nagar, Kolkata-700064, India 2 Gurudas College, Narikeldanga, Kolkata-700054, India 3 Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400005, India 4 Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India (Received 8 May 2007; published 20 August 2007) Excitation functions for the above-barrier fusion cross sections are measured for the first time for the 7 Li+ 28 Si system by two methods—the characteristic γ -ray method and the evaporation α measurement method—in the energy range E lab = 11.5–26 MeV. Experimental results are consistent and agree with each other, and the one-dimensional Barrier Penetration Model (BPM) predictions describe the data well up to twice the Coulomb barrier, but they overestimate the data by about 15–20% at higher energies. DOI: 10.1103/PhysRevC.76.027603 PACS number(s): 25.60.Pj, 25.70.Gh, 25.70.Jj Understanding the reaction dynamics of loosely bound stable projectiles at near-barrier energies have attracted much interest in recent years not only because of the new insights provided by them into the mechanisms but also because these nuclei are viewed as precursors to more exotic ra- dioactive nuclei with “haloes” or “skins.” Some of the novel experimental information obtained involving weakly bound nuclei are fusion enhancement or suppression, increase of breakup cross sections compared with tightly bound stable projectiles, weakening or absence of a threshold anomaly, or the occurrence of a new type of threshold anomaly related to the energy dependence of optical model potentials in the neighborhood of the barrier [1–10]. Theoretical studies [11–15] have also investigated the correlation between the weakly bound cluster structure of these nuclei and the dynamics of the reactions. These studies present a somewhat conflicting picture regarding the magnitude of fusion cross section (enhancement/suppression) at near-barrier energies and their dependence on the bombarding energy above the barrier. Besides the subbarrier and near-barrier fusion and the associated controversies, the scenario of fusion reaction involving weakly bound projectiles at above- barrier energies is also not clear and conclusive. One of the observations is that the breakup does not affect fusion [16] at above-barrier energies. The argument is that for breakup to affect fusion, it has to occur at a lower partial wave region. However, in this angular momentum region, breakup is followed by incomplete fusion (ICF) at higher energies, and the sum of complete fusion (CF) and incomplete fusion (ICF), i.e., the total fusion, remains unaffected. The total fusion (ICF+CF) agrees well with the well-accepted barrier penetration model prediction. This observation, however, does not really corroborate the results of Refs. [17–19]. The general finding in these investigations points toward an overall inhibition of fusion cross section at above-barrier energies, especially for light mass targets. It is to be emphasized, in this * harashit.majumdar@saha.ac.in context, that the ICF contribution for medium mass targets at above-barrier energies was found to be negligible [20]. There have been a number of complementary experimental investigations on scattering, e.g., for 6,7 Li+ 27 Al and 28 Si [21– 23], and on reaction and fusion, e.g., for 6,7 Li+ 27 Al [16,24], 6 Li+ 28 Si [25,26], 9 Be+ 27 Al, 28 Si [25,27], and 6,7 Li+ 59 Co [6], in the target mass range A ∼ 20–60. But these measurements are mainly confined to the Coulomb barrier probing the effect of breakup on scattering and fusion in the near-barrier energies. Very few of these data extend beyond twice the barrier energy. However, no fusion measurement exists for the 7 Li+ 28 Si system. We present in this article our measurement of fusion cross sections for the 7 Li+ 28 Si system at several energies extending from the Coulomb barrier to well above twice the barrier value. The total fusion excitation function of the 7 Li+ 28 Si system was measured using the characteristic γ -ray method. The experiment was carried out at the Bhabha Atomic Research Centre -Tata Institute of Fundamental Research (BARC-TIFR) 14UD Pelletron accelerator with a 7 Li (3 + ) beam at the energies E lab = 11.5, 12.5, 14, 16, 18, 20, 22, 24, and 26 MeV. Beam intensity was of the order of 5–20 pnA. A small thin- walled aluminum chamber was used to house the target, which consisted of 192 μg/cm 2 natural silicon sandwiched between two thin gold layers (40 and 100 μg/cm 2 ) to prevent oxidation and was prepared using a vacuum evaporation technique. The average energy loss in the target is about 200 keV. The characteristic γ rays emitted from the evaporation residues were detected using a Compton suppressed Clover detector placed at 55 ◦ with respect to the beam direction. Efficiency runs were taken at both the beginning and the end of the main experiment with a number of standard sources, i.e. 152 Eu, 133 Ba, and 207 Bi, spanning the energy range 85–1770 keV. The absolute efficiency in the add-back mode of the detector was measured with 152 Eu and 133 Ba standard radioactive calibrated sources placed at the target position. Target thickness was measured with the α energy loss method with a three-line α source, and the estimated uncertainty was about 5%. The background was measured at each energy with and without beam using a blank tantalum frame in place of the target. 0556-2813/2007/76(2)/027603(4) 027603-1 ©2007 The American Physical Society